1. Field of the Invention
The present invention generally relates to improvements in underwater pelletizers and more particularly to providing a water flow guide for the water box of an underwater pelletizer to more efficiently utilize pressurized water flowing through the water box. The water flow guide of this invention produces an accurately guided pressurized water flow which improves pelletization of polymers of various diameters, especially micropellets, and polymers having a wide range of specific gravity and rheological characteristics.
2. Description of the Prior Art
Underwater pelletizers are well known and include a die plate with multiple orifices through which molten polymer strands are extruded with the orifices terminating at a die face on the die plate. A powered rotary cutter including a cutter hub mounted on a shaft with a plurality of blades supported from the cutter hub is associated with the die face for shearing the extruded polymer strands into pellets as the polymer is extruded beyond the die face. A water box encloses the die face, cutter hub and blades to form a cutting chamber through which water flows to quench and rigidify the extruded polymer strands, thus enabling the cutting blades to better shear the extruded strands into pellets. The flow of water through the cutting chamber defined by the water box also conveys a slurry of the water and sheared pellets through the outlet of the water box to a pellet dewaterer and/or a dryer. The above described underwater pelletizers are disclosed in related U.S. patents including U.S. Pat. Nos. 4,123,207, 4,251,198, 4,621,996, 4,728,276 and 5,059,103, all owned by the assignee of this invention. Additional U.S. patents which relate to pelletizers are as follows:
U.S. Pat. No. 3,341,892 illustrates cutter hub assemblies directing water flow to the cutting area of the die plate. However, the water enters through the pelletizer shaft and is routed through the cutter hub with the water serving as an aid to pelletization. The water is not pressurized in the cutting chamber and gravity is relied upon to actually get the pellets into a solid stream of water to transport the pellets to the lower portion of the cutting chamber where the pellets exit. This patent also illustrates a flute style spray nozzle assembly that introduces a spray of water directed at the cutter blades where its purpose is to assist in so-called xe2x80x9chot facexe2x80x9d die plate pelletization.
U.S. Pat. No. 3,862,285 illustrates a cooling system for the production of plastic sheets incorporating a flow guide design. U.S. Pat. No. 4,245,972 illustrates the use of flow control that is actually designed to keep water away from the cut pellets until centrifugal force and gravity transport the pellets to a water ring formed within the bowl (formed at 14 and 10 in the patent).
U.S. Pat. No. 4,300,877 uses the cutter hub as an integral component of the flow control which could result in plastic contamination and cause tedious cleanup issues for operating personnel. U.S. Pat. No. 4,846,644 illustrates what is called a high speed xe2x80x9ccoolerxe2x80x9d and is essentially a water ring pelletizer somewhat similar to the structure disclosed in U.S. Pat. No. 4,300,877 and which does not use water to encapsulate/transport the pellets as they are cut.
U.S. Pat. No. 3,207,818 illustrates a cutting process where the specific gravity of the polymer plays an important role in operation. The xe2x80x9cchamberxe2x80x9d has no specific water (or process medium) flow guiding devices. The flow guide of this invention operates independently of any specific gravity considerations. U.S. Pat. No. 4,978,288 illustrates a water ring pelletizer where water is again isolated from the pelletizing process until well after cutting has taken place and U.S. Pat. No. 5,215,763 also illustrates a water ring pelletizer.
While the above listed patents disclose various structures associated with underwater pelletizers, the invention disclosed in the instant application utilizes the controlled flow of water not only to assist in cutting or shearing the pellets but also to quench and isolate pellets immediately after cutting has occurred. The type of pelletization known as xe2x80x9chot facexe2x80x9d, as shown in U.S. Pat. No. 3,341,892, uses an assist spray and is not underwater pelletization as utilized in the present invention. Underwater pelletization as in the present invention utilizes the pressurized water to encapsulate and isolate the pellets and also uses the pressurized water to transport the pellets away from the cutter hub.
The water flow guide of the present invention produces a pressurized water flow, accurately guided, to allow the underwater pelletization of polymers of nearly any diameter, even micropellets, i.e. pellets formed from polymer strands extruded through orifices having a diameter less than or equal to 0.050 inches, as well as polymers of a wide range of specific gravity/rheological characteristics. The flow guide within the underwater pelletizer relies on the water pressure/flow as opposed to gravity, such as is used in water ring pelletizers, and the pressurized water actually transports the pellets from the moment they are cut until they are introduced into a drying structure.
The water flow guide of the present invention is positioned within the cutting chamber defined by the water box of an underwater pelletizer in order to selectively guide the pressurized water passing through the water box. By selectively guiding the pressurized water in the cutting chamber, it has been found that the water can more efficiently quench the extruded polymer strands to cool and rigidify the strands, thus enabling the cutter blades to more effectively and cleanly cut or shear the extruded strands into pellets. Also by selectively guiding the pressurized water with the water flow guide of this invention it is possible to increase the efficiency of the pressurized water flowing through the water box to convey the sheared pellets away from the cutting chamber to the water box outlet and then to a pellet dewaterer and/or a pellet dryer.
In addition, the flow guide of this invention permits flow management of the pressurized water flow into and around the cutter within the cutting chamber. This leads to a reduction or even elimination, of cavitation which occurs when water cannot get into the central area of the cutter hub and die plate, thus reducing or eliminating pellet clusters or agglomerates forming in the cutting chamber, especially when producing micropellets. By reducing or eliminating pellet clusters or agglomerates, expensive shutdowns of the equipment and operator requirements and time to clean up and restart are significantly reduced. In addition, the wasting of polymer materials is reduced, all increasing productivity of the pelletizer. Moreover, if agglomerations and cutting chamber clog ups occur, the agglomerate or clog within the cutting chamber is easier to clean off and remove by the operator because there is no longer a large open cutting chamber to fill up with molten, but solidifying polymer. Furthermore, the reduction of cavitation allows the pelletizer to produce good quality, well sheared pellets at higher speeds and/or with more cutter blades, thus making it possible to yield higher rates of production, in pounds/hour or kilograms/hour, which results in a higher capacity for the same size pelletizer.
Furthermore, it is believed that the flow guide of the present invention allows the pelletizer to operate with less load, and thus pulls less amperes by the motor which, in turn, allows the pelletizer to operate more efficiently with less power requirements. Also, in utilizing the water more efficiently with the flow guide of the present invention, less water flow is required for the pelletizer. Less water flow translates into smaller pumps and/or less pump energy consumption.
The water flow guide of this invention is oriented in the cutting chamber between the water inlet and the water and pellet slurry outlet of the water box which are typically disposed in diametrically opposed relation, such as a water inlet at the bottom of the cutting chamber and the water and pellet slurry outlet at the top. The water flow guide effectively directs the pressurized cooling water toward the die face and the polymer strands being extruded from the die face. By directing the water flow to the die face, it has been found that the water more efficiently cools and rigidify the polymer strands in order that the rotating cutting blades can shear the extruded polymer strands with a straight diametric cut. Also, effective direction of the pressurized water flow in association with the die face will more effectively entrain the sheared pellets in the water flow thereby more efficiently conveying the slurry of water and pellets to the outlet in the water box which enables the slurry to be introduced into a pellet dewaterer or pellet dryer.
The water flow guide is stationarily mounted within the cutting chamber, preferably on the front of the pelletizer. The flow guide is in the form of a cylindrical tubular member affixed to the pelletizer in generally concentric relation to the rotating shaft of the pelletizer, preferably with the body of the guide filling out the majority of the cutting chamber volume. The free end of the guide tubular member preferably terminates adjacent to the rotating cutter hub and cutting blades mounted thereon.
In a first embodiment of the invention, the water flow guide is a generally cylindrical tubular member with the external surface thereof being disposed adjacent to the generally cylindrical inner surface of the water box, preferably with a very small clearance therebetween. The tubular member includes a generally cylindrical axial bore which is positioned over and surrounds the rotating cutting hub shaft forming an annular space therebetween. The cylindrical outer surface of the water flow guide includes diametrically opposed axially extending recesses with one recess in alignment with the water inlet and the other recess in alignment with the slurry outlet of the water box. One end of each of the axial recesses is spaced from the ends of the cylindrical surface and in registry with the water inlet and slurry outlet, respectively. The other end of each of the axial recesses opens to a free end of the water flow guide which terminates adjacent the cutter hub and cutter blades.
As such, the flow of water is directed axially from the water inlet toward the die face and the flow of water and pellets is directed axially away from the die face and cutter hub toward the slurry outlet. Water flows from the axial recess in registry with the water inlet across the die face for more effective heat exchange engagement with the polymer strands being extruded from the die face to more efficiently cool and rigidify the extruded polymer strands which enables the extruded strands to be more effectively sheared into pellets by the cutter blades. The water and sheared pellets then are discharged along the axial recess in registry with the outlet in the water box as a slurry of pellets and water. The slurry enters the open end of the axial recess in registry with the outlet and moves axially in the recess in the outer surface of the water flow guide and then out through the outlet in the water box.
In a second embodiment of the invention, the water flow guide also comprises a generally cylindrical tubular member with an outer wall, again preferably, sized to be only slightly smaller than the inner wall of the water box to provide a very small clearance between the adjacent surfaces. However, the tubular member in this embodiment has a radial opening forming a tubular radial extension of the pressurized water inlet which terminates in the internal annular space defined by the cylindrical axial bore spaced concentrically from the cutter shaft of the pelletizer. An open end of the cylindrical axial bore terminates in spaced but adjacent relationship to the cutter hub and cutter blades to enable incoming pressurized water to be discharged from the open end of the cylindrical bore toward the cutter hub, cutter blades and die face. The pressurized water flow in the annular space between the shaft and the cylindrical inner wall of the axial bore flushes pellets from this area to keep it clear of pellets and can serve to prolong the life expectancy of the pelletizer shaft seals. This embodiment of the water flow guide also includes an external cylindrical surface having an axial recess in alignment with the outlet of the water box similar to the corresponding recess in the first embodiment. One end of the axial recess is in registry with the outlet. The axially extending recess includes an open end terminating adjacent the cutter hub to provide a flow passage for the slurry of water and pellets being discharged from the pelletizer.
In a third embodiment of the invention, the water flow guide again includes the generally tubular cylindrical member as described in the first and second embodiments. The cylindrical member also has a pair of diametrically opposed recesses aligned with and in registry with the pressurized water inlet and outlet in the water box in a manner similar to the recesses as described in the first embodiment. However, in this embodiment, the free end of the water flow guide is sloped or beveled so that the bottom portion of the right side of the free end of the water flow guide is spaced from the die face a distance less than the upper portion of the right side. The left side of the free end is oppositely sloped. Therefore, water entering the water box and passing through the axial recess toward the die face encounters less resistance to flow as it enters a larger volumetric area between the bottom portion of the free end of the water flow guide and the die face. The slope or bevel and the counterclockwise rotation of the cutter hub and cutters at relatively high rotational speed provides less resistance to water flow to the left side of the water flow guide before getting to the area in which the cutter hub and cutter blades are rotating thereby avoiding the tendency of the majority of water flow occurring across one-half of the water flow guide and cutter hub and cutter blades. The right side of the water flow guide is more restrictive to water flow immediately as it travels across the water inlet area. As the rotating cutter hub and cutter blades are rotating and cutting plastic pellets, the total flowing mass consisting of water and pellets is increasing. The additional free flow area in the water flow guide accommodates the pellets being added to the water flow stream as they are being sheared from the polymer strands at the die face.
In a fourth embodiment of the invention, the water flow guide includes a tubular cylindrical member constructed in accordance with the third embodiment with the addition of a radially extending hole extending straight inwardly from the inner surface of the axial recess in registry with the pressurized water inlet. The inner end of the radial hole terminates at the surface of the axial bore of the tubular member which is spaced concentrically around the cutter shaft. This enables a portion of the pressurized water to flow around the cutter shaft in order to flush the area around the cutter shaft. This circular flow around the cutter shaft removes any cut pellets that may have a tendency to pack into this area. Maintaining the area around the cutter shaft free of pellets can prolong the life of cutter shaft seals.
In each embodiment of the invention, pressurized water inflow is directed axially toward the die face, the cutter hub and cutter blades for more effective heat exchange contact between the cooling water and strands of polymer extruded from the die face to cool and rigidify the strands of polymer for more efficient shearing of the polymer strands by the cutting blades. Also, the slurry of water and polymer plastics are more effectively moved away from the rotating cutter hub and cutter blades and discharged axially toward the outlet of the water box.
Accordingly, it is an object of the present invention to provide a water flow guide positioned in the cutting chamber of an underwater pelletizer to guide pressurized water flow from a water inlet, axially toward the die face for more effective heat exchange contact with the polymer strands being extruded through the die plate in order for the rotating cutter blades to more effectively shear the polymer strands into pellets. The pressurized slurry of pellets and water flow is then directed away from the die face radially and axially to a water and pellet slurry outlet for subsequent separation of pellets and water and drying of pellets. The selectively guided pressurized water flow assists in quenching the polymer strands, shearing the strands into pellets and transporting the pellets after shearing.
Another object of the present invention is to provide a water flow guide associated with the interior of a cutting chamber of an underwater pelletizer for establishing an axial flow path for incoming water from a lateral or radial inflow to direct water toward the cutter hub, cutter blades and die face to more efficiently cause heat exchange contact with the extruded polymer for cooling and rigidifying polymer strands extruded from the die face for more effective shearing of the strands into pellets with the flow guide also establishing an axial flow path for discharge of a slurry of pressurized water and pellets to a radial or lateral discharge outlet in a water box of an underwater pelletizer.
A further object of the present invention is to provide a water flow guide stationarily positioned in the cutting chamber of an underwater pelletizer in which the flow guide with the tubular member defines passages for circulation of incoming pressurized water toward and around the die face, cutter hub and cutter blades for efficient heat exchange contact with the polymer strands as they are extruded and sheared at the die face and then axially away from the die face to a discharge for more efficiently discharging the slurry of water and sheared pellets.
Another object of the present invention is to provide a water flow guide in accordance with preceding objects in which the water flow guide comprises a generally tubular member which includes diametrically opposed axial recesses in the external surface thereof extending from an end of the tubular member adjacent the cutter hub and cutter blades axially to a point in alignment with the water inlet and slurry outlet of the cutting chamber.
A still further object of the present invention is to provide a water flow guide in which the tubular member includes an axial bore having an internal surface spaced from the shaft drivingly supporting the cutter hub and cutter blades with the tubular member including an inlet extension communicating the pressurized water inlet of the cutting chamber with the internal surface of the tubular member so that inlet water can flow axially around the drive shaft toward the cutter hub and die face to maintain the space between the internal surface of the guide and shaft free of pellets.
Still another object of the present invention is to provide a water flow guide associated with the water box, cutting chamber, cutting hub and cutting blades to define a water flow path oriented across the die face for more efficient use of the water in cooling and rigidifying polymer strands extruded from the die face and more efficient use of the water in entraining and conveying pellets sheared from the strands to the outlet of the cutting chamber.
Yet another object of this invention to be specifically enumerated herein is to provide a water flow guide in accordance with the preceding objects and which will conform to conventional forms of manufacture, be of simple construction and easy to use so as to provide a device that will be economically feasible, long lasting and relatively trouble free in operation.